1. Field of the Invention
This invention relates generally to apparatus, such as a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape and, more particularly, is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During the recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, means are provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter. In published Japanese Patent Application No. 9919/1974 (Applicant: Matsushita Electric Industrial Company, Limited), it is generally disclosed to control the amount of deflection of the head in the direction normal to its plane of rotation in dependence on the difference between the speeds of advancement of the tape during the recording and reproducing operations so as to theoretically make possible the correct reproduction of the recorded video signals in the still-motion mode in which the tape is at rest, in the slow-motion mode in which the speed of advancement of the tape is, for example, 1/4 or 1/2 the tape speed for the recording operation, and in the quick-motion mode in which the speed of advancement of the tape is substantially greater than, for example, 2 times, the tape speed, for the recording operation. Further, in published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), it is disclosed to detect the amount of deflection of the head required for accurately scanning a record track thereby, and, when the detected deflection approaches the physical limit of that possible with the bi-morph leaf or other transducer deflecting device supporting the head, the electrical drive signal for the bi-morph leaf is controlled to cause the head to scan the next adjacent record track, that is, to effect a so-called "track jump". By reason of such "track jump" it is theoretically possible to effect correct slow-motion reproduction and also reverse-motion reproduction in which, for example, the tape is advanced longitudinally at the same speed as for the recording operation, but in the reverse or opposite direction.
However, in many existing tracking control or servo systems, the amplitude of the deflection of the rotary head or transducer by its transducer deflecting device is not optimized, that is, the maximum required deflection of the head in the non-normal reproduction modes, for example, the still-motion, slow-motion, fast-forward and reverse-motion modes, is not minimized. The foregoing tends to limit the permissible tape speed for reproducing in the fast-forward and reverse-motion modes, and to cause phase deviations or errors in the reproduced signals. Further, the failure to optimize the amplitude of the deflection of the rotary head or transducer by the bi-morph leaf or other transducer deflecting device disadvantageously decreases the durability of the transducer deflecting device and the speed and linearity of its response to the electrical drive or control signal therefor. Moreover, a circuit of undesirably large capacity is required for providing the electrical drive signal to the transducer deflecting device when the deflection amplitude is not optimized.
In U.S. Patent Applications Ser. Nos. 06/073,246 and 06/152,117, filed Sept. 6, 1979 and May 21, 1980, respectively, which have matured into U.S. Pat. Nos. 4,287,538 and 4,361,857, respectively, and both having a common assignee herewith, there is disclosed an automatic tracking control system which optimizes the amplitude of the deflection of the rotary head by its deflecting device for each of the non-normal reproducing modes. More particularly, in the foregoing automatic tracking control systems, the drive signal applied to the transducer deflecting device comprises a head position and track selection control signal for determining the starting position of the transducer or head and, thereby, the next desired track to be followed or scanned thereby, a dithering signal or oscillation, and a track following error signal.
With a helical scan VTR, vertical synchronizing signals are recorded, for example, at the end of each track on the tape, so that each vertical synchronizing signal corresponds to one of the record tracks extending obliquely on the tape. The vertical synchronizing signals are compared with external reference signals obtained from rotation of the guide drum in the aforementioned commonly assigned applications to provide the head position and track selection control signal. However, the situation may occur where one or more vertical synchronizing signals are dropped out or generated at an early or later time, for example, due to tape tension variation or damage of the tape itself. During reproduction in the normal reproducing mode, such dropping out or variation in the vertical synchronizing signals does not adversely affect the reproduced picture. This is because an external reference vertical synchronizing signal can be substituted for the dropped out vertical synchronizing signal so that proper processing of the video signals by a time base correcting circuit can occur.
A problem occurs, however, during the special reproducing modes, such as the slow-motion, still-motion and quick-motion modes. In such case, the reproducing transducer or head does not always follow successive record tracks on the magnetic tape. Instead, the reproducing head is caused to jump, that is, skip over, one or more record tracks in the quick-motion mode, and to reproduce the same track one or more times in the still-motion and slow-motion modes. In such case, the vertical phase of the field corresponding to a track being reproduced varies in accordance with the amount and direction of the head jump. Accordingly, if a vertical synchronizing signal is dropped out during reproduction in a special mode, a problem occurs with specifying the vertical phase of the field corresponding to the track then being reproduced, resulting in a loss in continuity of the vertical phase of the reproduced video signal. This results in problems in the operation of the time base correcting circuit with a consequent deterioration in the reproduced picture.